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Abstract

We present a technique for enhancing the light output power of quantum cascade lasers (QCLs) by tilting of the front facet, which leads to a change of the modal reflectivity, resulting in an asymmetric light intensity distribution along the laser cavity. This asymmetry provides most of the light being emitted through one facet of the laser. An experimental study of threshold current, slope efficiency and light output power as a function of the front facet angles were performed and compared to conventional QCLs. The lasers with a front facet angle of 8° shows a 20% improved power output from the front facet.

Figures (7)

Sketch of the (a) symmetric and (b) asymmetric light intensity distribution inside a laser cavity. The arrows (dash) and arrows (line) are indicating the light reflection and transmission at the laser facet.

Sketch of the asymmetric light intensity distribution inside a laser cavity for the tilted front facet. The arrows (dash), arrows (line) and symbols ( × —–) are indicating the light reflection, emission at the laser facets and reflected light at the tilted front facet which is not coupled back into the cavity, respectively.

(a) Sketch of a QCL with a tilted front facet. Only the front facet of the QCLs was milled, while the back facet was left as cleaved. (b) A scanning electron microscope picture shows the fabricated tilted front facet (θF = 12°). ( + ) and (–) signs are defined for the far field measurement in the following section.

(a) The ratio of the threshold current densities of QCLs after FIB milling to before FIB milling for all given facet angles. (b) The experimentally determined modal reflectivity (triangles) of the tilted facet as a function of the facet angles.

(a) A comparison of LIV characteristics (the linear part of the LI curve just above threshold) for the emission from front and back facets of the QCL with a facet angle of 17°. (b) Ratios of the slope efficiencies between front and back facet (triangles), as a function of facet angles compared to the ratios (Eq. (2) of the power arriving at the laser facets (stars).

(a) A comparison of LJV characteristics between before and after FIB milling for QCLs with a facet angle of 8°. (b) The ratios, between before and after FIB milling, of ηs for the QCLs as a function of the facet angles, based on the emitted light from the front facet.

(a) Far field profiles measured along the lateral (Y) direction [see Fig. 3(a)] of various facet angles of QCLs, driven nearly at the peak optical power. The dash line at 0° represents the ridge normal direction. (b) The beam emission angle from the ridge normal (circles) and FWHM (squares) as a function of the facet angles are shown.